Electrophotographic technology has been widely applied to the field of printers, as well as the field of copiers, due to its immediacy and formation of high-quality images. Electrophotographic photoreceptors (hereinafter, optionally, referred to as “photoreceptor”) lie in the core technology of electrophotography, and organic photoreceptors using organic photoconductive materials have been developed, since they have advantages such as non-pollution and ease in production in comparison with inorganic photoconductive materials.
In general, an organic photoreceptor is composed of an electroconductive substrate and a photosensitive layer disposed thereon. Photoreceptors are classified into a so-called single-layer photoreceptor having a single photosensitive layer (single photosensitive layer) containing a binder resin dissolving or dispersing a photoconductive material therein; and a so-called multilayered photoreceptor composed of a plurality of laminated layers (laminated photosensitive layer) including a charge-generating layer containing a charge-generating material and a charge-transporting layer containing a charge-transporting material.
In the organic photoreceptor, changes in use environment of the photoreceptor or changes in electric characteristics during repeated use may cause various defects in an image formed with the photoreceptor. In a method as one technique for solving such disadvantages, an undercoat layer containing a binder resin and titanium oxide particles is provided between an electroconductive substrate and a photosensitive layer in order to stably form a good image (for example, refer to Patent Document 1).
The layer of the organic photoreceptor is generally formed by applying and drying a coating liquid prepared by dissolving or dispersing a material in a solvent, because of its high productivity. In such a case, since the titanium oxide particles and the binder resin are incompatible with each other in the undercoat layer, the coating liquid for forming the undercoat layer containing titanium oxide particles and the binder resin is provided in the form of a dispersion of titanium oxide particles.
Such a coating liquid has generally been produced by wet-dispersing titanium oxide particles in an organic solvent using a known mechanical pulverizer, such as a ball mill, a sand grind mill, a planetary mill, or a roll mill, by spending a long period of time (for example, refer to Patent Document 1). Furthermore, it is disclosed that when titanium oxide particles are dispersed in a coating liquid for forming an undercoat layer using a dispersion medium, an electrophotographic photoreceptor that exhibits excellent characteristics in repeated charging-exposure cycles even under conditions of low temperature and low humidity can be provided using titania or zirconia as the dispersion medium (for example, refer to Patent Document 2).
Phthalocyanines having photoconductive characteristics exhibiting highly sensitive to light with a long wavelength have been extensively studied as an excellent photoconductive material. In particular, phthalocyanines can be suitably applied to electrophotographic photoreceptors, plate-making materials in electrophotographic systems, or photoelectric transducers such as an image sensor, and are used as charge-generating materials of electrophotographic photoreceptors for long-wavelength semiconductor lasers or light-emitting diodes.
With phthalocyanines, it is known that physical properties such as absorption spectrum and photoconductivity vary depending on the type of the central metal and the physical properties significantly vary depending on its crystal form. Among phthalocyanines, for example, oxytitanium phthalocyanine and hydroxygallium phthalocyanine have highly sensitive photoconductive characteristics and are present in various crystal forms.
Among them, type V hydroxygallium phthalocyanine and type D crystalline oxytitanium phthalocyanine, which show distinct peaks near a Bragg angle (2θ±0.2°) of 27° to 29° in a powder X-ray diffraction spectrum to CuKα characteristic X-rays, exhibit high sensitivity (for example, refer to Patent Documents 3 and 4).
It is also known that so-called type D crystalline oxytitanium phthalocyanine exhibits significantly high sensitivity (for example, refer to Patent Document 3).
Furthermore, it is known that type D oxytitanium phthalocyanine shows a strong diffraction peak in a Bragg angle (2θ±0.2°) of 9.0° to 9.8° in a thin-layer X-ray diffraction spectrum to CuKα characteristic X-rays (for example, refer to Patent Documents 5 to 7).
In some production processes of oxytitanium phthalocyanine, titanium chloride or a chlorinated organic compound is used. As a result, the obtained oxytitanium phthalocyanine crystals may contain chlorine (for example, Patent Document 8).    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 11-202519    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 6-273962    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 10-67946    [Patent Document 4] Japanese Unexamined Patent Application Publication No. 2-8256    [Patent Document 5] Japanese Patent No. 2881921    [Patent Document 6] Japanese Patent No. 2502404    [Patent Document 7] Japanese Unexamined Patent Application Publication No. 2000-7933    [Patent Document 8] Japanese Unexamined Patent Application Publication No. 2001-115054